In many situations it is desirable to determine the temperature of a plate by non-contact arrangements. This is particularly the case in determining production worthiness of semiconductor wafers undergoing processing during the manufacture of high-density integrated circuits. The semiconductor wafer, i.e., substrate, usually formed of silicon or a III-V or II-VI compound such as gallium arsenide or zinc telluride, is typically in a high-vacuum environment, usually containing corrosive gases. This environment frequently has an adverse effect on conventional temperature monitoring probes that are in physical contact with the wafer. Contact between the probe and wafer causes wafer defects in the vicinity of the contact area. Such contact causes high-density circuitry near the contact area to be essentially destroyed, thereby reducing wafer yield and the number of chips obtained from the wafer.
To avoid the problems associated with temperature probes being in contact with the wafer, optical pyrometers are frequently used to monitor wafer temperature. To provide an accurate indication of substrate temperature from an optical pyrometer, the emissivity of the substrate must be known. With currently available pyrometer techniques, it is very difficult, if not impossible, to ascertain accurately emissivity of a semiconductor wafer undergoing processing for manufacture of integrated circuits. Generally, emissivity is approximated for the particular environmental conditions. This is not usually an accurate solution.
At temperatures below approximately 600.degree. C., undoped silicon is very transparent to infrared energy. As the temperature or doping level of the wafer increases, the wafer becomes less transparent to infrared energy. The decrease in transparency causes the emissivity of radiant energy that can be detected by an optical pyrometer to change in a fairly unpredictable manner. Emissivity of optical energy from the wafer is also dependent on how rough the wafer emitting face is. In addition, wafer emissivity as a whole is a function of material deposited on the face of the substrate in the optical pyrometer field of view. Since emissivity from the substrate is variable for the reasons mentioned, the output of the optical pyrometer is frequently not an accurate indication of wafer temperature. Despite the known inaccuracies of optical pyrometers, these instruments are generally preferred to the more accurate measurements attained with contact techniques.
It is, accordingly, an object of the present invention to provide a new and improved more accurate non-contact apparatus for and method of measuring temperature of a plate non-opaque to optical energy, such as a semiconductor wafer undergoing processing.
Another object of the invention is to provide a new and improved non-contact method of and apparatus for determining temperature of a plate non-opaque to optical energy.
Another object of the invention is to provide a new and improved apparatus for and method of monitoring the temperature of a semiconductor wafer undergoing processing in a vacuum, chamber with use of only non-contact optical equipment.
A further object of the invention is to provide a new and improved non-contact method of and apparatus for monitoring temperature of a plate non-opaque to optical energy, which plate has unpredictable variations in emissivity as functions of plate properties and the environment in which the plate is located.